1. Field of the Invention
This invention relates in general to communication networks and in particular to an apparatus, system, and computer program for synchronizing communications in a satellite communications network by accurately determining an arrival time of a signal from a remote terminal.
2. Discussion of the Background
Geo-synchronous satellite communication networks have existed for decades in various topologies and using various methods for sharing a fixed bandwidth channel between multiple users (Pritchard, Wilbur L., and Joseph A. Scivlli, Satellite Communication Systems Engineering, Prentice-Hall, 1986, incorporated herein in its entirety by reference). As these networks evolved, Time Division Multiple Access (TDMA) and Frequency Division Multiple Access (FDMA) bandwidth sharing techniques, and bandwidth efficient Quadrature Phase Shift Keying (QPSK) modulation have become de facto standards for Layer 1 (physical layer) although many other techniques and modulations are used to some extent.
Pure TDMA mesh networks exist, in which all remotes (user nodes) take turns transmitting in a half duplex fashion. Pure FDMA point-to-point, or Single Channel Per Carrier (SCPC) networks, exist to allow full duplex transmission and reception. Many commercially viable network systems have evolved into a hybrid star topology, taking advantage of the broadcast nature of geo-synchronous satellites by using a SCPC downstream carrier from a central hub to all user nodes, and using one or more TDMA upstream(s) shared by all user nodes to communicate with the hub.
Conventional satellite communication networks include a central control server 10 (hub) and a plurality of remote terminals 14 (remotes) that exchange information via a geo-synchronous satellite 12 (satellite) as shown in FIG. 1. The hub sends information to the remotes using Single Channel Per Carrier (SCPC) method and the remotes respond to the hub using Time Division Multiple Access (TDMA) method.
The hub includes a signaling protocol that conveys channel allocation assignments between all remotes and the hub to determine how many transmission opportunities to grant each remote during an upcoming time interval according to a set of pre-configured rules. A synchronization process guarantees that messages sent via this signaling protocol are received and applied by all remotes during the same time interval such that communication parameters may be changed at least as often as every time interval. The time intervals may be as short as 40 ms (plus processing delay) for real time operations.
A popular and simple implementation of the TDMA upstream channel is to use an ALOHA technique enabling any remote to transmit to the hub any time the remote has data to send and relying on the probability that no other remote chose to transmit at that time which would cause a collision. Once the demand for bandwidth exceeds a predetermined percentage of the upstream channel, a more complex methodology is required to schedule TDMA bursts so that collisions are prevented.
However, because the satellite is a “mirror” that redirects data coming from the remotes to the hub or viceversa, to synchronize communications it may be necessary that a time synchronization of the data coming from the plurality of remotes is achieved before that data arrives at the hub or otherwise the hub is not able to “understand” that data. In other words, in the TDMA method, different slots are allocated to each remote such that no two remotes send data to the hub at the same time.
Even with a control application that assigns to each remote unique slots in a frame for communicating with the hub, collision of data from multiple remotes arriving at the same time at the hub is possible. One cause of this collision is the fact that the physical movement of the geo-stationary satellite is not taken into account by the conventional control applications that determine and allocate the slots in the frame for each remote.
Although the satellites are geo-stationary, meaning that the satellites are relatively fixed above a particular position above the Earth, in reality, each geo-stationary satellite moves randomly inside an imaginary box having a size given by multiplying the distance from the ground to the satellite with an angle of 0.01° measured in radians. This movement of the satellite introduces unexpected and unaccounted delays, which might result in a loss of communication between the hub and the remotes.
FIG. 2 illustrates additional problems with conventional network communication methods, as recognized by the present inventors. This figure represents two timing scenarios, labeled as follows: No Synchronization, and Conventional Synchronization. In each scenario, HRxSOFn(ta) represents the nominal time a start of a data burst time frame (i.e. Start of Frame (SOF)) is received from remote “n,” and kmax*2 represents the maximum variation of reception time, or tracking error, for the worst case remote, due to minor perturbations in the satellite position. The frame's relative frame sequence is “a.”
In the No Synchronization scenario, SOFs are unconstrained and arrive at various times due to variations between the propagation delays of each remote. In the Conventional Synchronization scenario, a time Rcdn (Remote Conventional Delay for User Node n) is added at each remote so that SOFs from all remotes are received synchronously to the Hub's transmit reference SOF.
The conventional approach synchronizes HRxSOFn of each remote, but arrival time variations resulting from satellite tracking errors disadvantageously results in some of HRxSOFn occurring during hub frame n and some of HRxSOFn occurring during hub frame n-1.
Further, in the conventional approach, Rcdn is chosen to synchronize HRxSOFn with subsequent Hub transmit SOF, which results in simultaneous arrival of SOFs sent at different times in the frame sequence. Thus, data bursts arriving at the Hub cannot be assumed to have all been sent during the same frame time, thereby complicating network control methods and increasing response time to control commands sent from the Hub. In addition, neither approach takes into consideration the movement of the satellite relative to the hub and the remotes.
The industry is in need of a network system that combats these problems and enables standard off-the-shelf networking equipment using standard protocols to take advantage of a communication satellite's ability to reach remotes at extreme distances and in areas of the world where broadband terrestrial communication is not practical, irrespective of the delays introduced into the satellite communication network by various of its components or the movement of the satellite.